1. Field of the Invention
The present invention relates to a torque converter including a controlling device for a lock-up clutch capable of direct power transmission between an input side and an output side, and, more particularly, to a torque converter which controls a power transmission capacity (engagement capability of the lock-up clutch) in accordance with, for example, a running state of a vehicle.
2. Description of the Related Art
Various types of technologies have been proposed for preventing variations in an engagement time or generation of shock when a lock-up clutch is engaged in accordance with the state of a vehicle, in controlling the hydraulic pressure of the lock-up clutch. For example, Japanese Examined Patent Application Publication No. 7-113409 and Japanese Unexamined Patent Application Publication No. 2001-116138 each disclose a lock-up clutch hydraulic controlling device which feeds back the internal pressure of a torque converter chamber to a pressure control valve and adds a bias equivalent to the internal pressure of the torque converter to the hydraulic pressure of a lock-up clutch. In addition, for example, Japanese Unexamined Patent Application Publication No. 2001-271906 discloses a technology in which a conduction pipe bypassing an inlet flow path and an outlet flow path of, for example, a torque converter is disposed, a pressure sensor is disposed at an intermediate portion of the conduction pipe, the internal pressure of the torque converter chamber is estimated, and the estimated value is used for controlling the lock-up hydraulic pressure.
In particular, when performing a slip control operation on a lock-up clutch at a target slip amount, in order to eliminate the effect of a difference between the time that a fluid pressure is supplied and the time that the lock-up clutch actually operates, it is desirable to perform, for example, a pre-charge control operation before a lock-up piston (hereafter referred to as “piston”) operates. Here, it is necessary to precisely determine the back pressure of the piston (that is, the pressure acting in a direction opposing the direction of operation of the piston) and to reflect the determined back pressure in the lock-up pressure. However, it is difficult to precisely detect and predict the back pressure of the piston in an actual vehicle.
The aforementioned related technologies focus attention on the fact that the internal pressure of the torque converter acts upon the back side of the piston. With the internal pressure being assumed as being equivalent to the back pressure of the piston, inlet and outlet flow paths of the torque converter communicate with a lock-up pressure regulating valve in order to feed back the internal pressure of the torque converter. FIG. 8 is a hydraulic circuit diagram of a lock-up controlling device for the torque converter, disclosed in Japanese Unexamined Patent Application Publication No. 2001-116138. In FIG. 8, an inlet pressure and an outlet pressure of the torque converter are made to act upon a valve 22 for producing a lock-up control pressure.
As correctly pointed out in the related art, the inlet pressure and the outlet pressure of the torque converter do greatly vary with the temperature of an operating fluid and the rotation of a turbine. Strictly speaking, for example, the structure of and the flow in a hydraulic path and centrifugal hydraulic pressure affect the back pressure of the piston. FIG. 9 shows a graph of the results of actual measurements of the back pressure of a piston and an inlet pressure and an outlet pressure (T/C in and T/C out) of a torque converter in terms of different rotational speeds Nt of a turbine. Whereas the inlet pressure and the outlet pressure of the torque converter increase as the rotational speed Nt increases, the back pressure at 3000 rpm is less than the back pressure at 2000 rpm. Therefore, in the method that makes use of the inlet pressure and the outlet pressure of the torque converter, feedback is sometimes not properly performed.
For example, in the technology disclosed in Japanese Examined Patent Application Publication No. 7-113409, the internal pressure of the torque converter is fed back, and the hysteresis occurring when the lock-up pressure increases or decreases is relatively large. Therefore, this technology is not suitable for continuously performing slip control operations at target rotational speeds.
In addition, for example, in the technology disclosed in Japanese Unexamined Patent Application Publication No. 2001-271906, due to, for example, the effects of the structure of and flow in the hydraulic path, a highly precise back pressure cannot be obtained on the basis of an estimated internal pressure of the torque converter. Even if the estimated value is used in fine controlling of the lock-up pressure in accordance with the state of a vehicle, a shock is produced when piston contact occurs.